Method for counteracting the occlusion effects

ABSTRACT

A method for counteracting the occlusion effect of an electronic device delivering an audio signal to the ear, like a hearing aid or an active ear protector, where the electronic device includes a transmission path with an external microphone or input line which receives a signal from the environment and an signal processor and a receiver which receives processes signal from the signal from the signal processor and delivers sound signals to the ear, whereby an ear piece is inserted into the ear canal and totally or partially blocks the canal. The sound conditions in the cavity between the ear piece and the tympanic membrane are directly or indirectly determined, and whenever condition leading to occlusion problems are determined, the transmission characteristics of the transmission path or the receiver changes in order to counteract the occlusion effect.

AREA OF THE INVENTION

This invention relates to a method for counteracting the occlusioneffect of a sound protector and/or communication device like a hearingaid, whereby an ear piece is inserted into the ear canal and totally orpartially blocks the canal.

BACKGROUND OF THE INVENTION

One of the most common complaints of hearing aid users is that their ownvoice sounds unnatural: boomy, hollow or echoing. Poor sound quality ofa person's own voice is also one of the top ten reasons why some hearingaids end up in the drawer. This problem with a person's own voice isvery often due to the so-called occlusion effect which occurs becausethe body-conducted contribution to a person's perception of his/her ownvoice is trapped in the cavity between the occluding earmold of thehearing instrument and the tympanic membrane. The result is a build-upof sound pressure at low frequencies that may be as much as 30 dBrelative to the open-ear. Typically, the occlusion effect has a flatmaximum between 80-500 Hz and vanishes above 1 kHz. In the open-earcondition and at the low frequencies considered here, the body-conductedcontribution is insignificant compared to the air-conductedcontribution. In today's hearing aid dispensing there are basicallythree ways to address the client's eventual occlusion problem withhis/her own voice. First, the earmold (or ITE hearing aid) may beequipped with a vent through which the body-conducted part of his/herown voice can dissipate. Secondly, it has been shown that CICinstruments that are fitted with a seal in the bony part of the earcanal can solve or at least reduce the occlusion problem in many cases.Unfortunately, bony sealed CICs have earned a bad reputation forintroducing physical discomfort and are hence rarely dispensed. Thirdly,occlusion problems may be dealt with by counseling—along the lines of“You'll get used to it!”. A number of hearing aid users do not manage toget used to it, and they prefer to live with their hearing disorderun-aided.

In U.S. Pat. No. 4,985,925 an active noise reduction based on a negativefeed back electro-acoustical system is shown. The system consists of anelectronic earplug seated in the concha fossa combining active andpassive noise reduction in the quiet zone at the ear, a bilateraltransducer circuit which drives a speaker as an acoustical velocitysource, a shunt feed back control filter network which improvesstability and increases noise reduction, and a combined inputnoise-filter/feed back system. A typical application is in a noisyenvironment for hearing protection and for improved communicationcapability.

SUMMARY OF THE INVENTION

The invention seeks to provide a real solution to the occlusion problemby making use of active hearing aid components.

This is achieved in a method for counteracting the occlusion effect ofan electronic device delivering an audio signal to the ear, like ahearing aid or an active ear protector. The electronic device includes atransmission path with an external microphone or input line whichreceives a signal from the environment and a signal processor and areceiver which receives a signal from the signal processor and deliverssound signals to the ear, whereby an ear piece is inserted into the earcanal and totally or partially blocks the canal. The sound conditions inthe cavity between the ear piece and the tympanic membrane are directlyor indirectly determined, and whenever conditions leading to occlusionproblems are present, the transmission characteristic of thetransmission path to the receiver counteracts the occlusion effect.

Keeping track of the sound conditions in the cavity of the occluded earcanal can be done in a number of different ways and the chosen way isnot crucial to the invention. Also counteracting the occlusion can bedone in a number of different ways by appropriate choice of transmissioncharacteristic of the transmission path from the input to the receiver.

In an embodiment of the invention the conditions leading to occlusionproblems are determined by monitoring the activity of the user's ownvoice, and when a user's own voice activity is detected, theamplification through the signal processor in the frequency region below1 kHz is reduced. It is the sound transmission through the tissue of thesound from a user's own voice which often leads to the sound pressurebuild up in the cavity. This can be compensated for by reducing theamplification through the hearing aid in the relevant frequency regionbelow 1 kHz. Hereby, the total sound pressure level in the cavitybecomes comfortable. There are a number of ways in which a user's ownvoice activity can be monitored. One way is to analyze the input signalfrom the usual microphone and to determine when characteristics whichare special to the user's voice are present in the signal. Also, it ispossible to use a vibration monitor which monitors the level ofvibration in the tissue adjacent to the ear piece. Possibly, thevibration monitor is built into the ear piece.

The sound conditions in the cavity can be monitored by an additionalmicrophone, which is acoustically coupled to the cavity. The signal fromthe additional microphone is used in a feed back loop to the receiver inorder to attenuate the low frequency part of the sound in the cavity.The feed back loop attenuates all low frequency sounds regardless ofwhether they stem from body functions such as chewing or from own voiceor from another source.

When the occlusion problem is solved as described above the attenuationof the low frequency parts of the sound also is applied to the sound,which is received from the surroundings, and this is not desirable. Thiscan be overcome by having the signal processor amplify the low frequencypart of the signal from the external microphone in order to compensatefor the attenuation of the useful part of the signal from the externalmicrophone or input line. In this way the useful low frequency parts ofthe signal, which are attenuated by the feed back loop, may be restoredin the signal processor. Thus, the user gets the sound from thesurroundings with the usual amplification while the occlusion effect isremoved or reduced.

According to an embodiment of the invention the feed back loop from theadditional microphone is activated by a user's own voice activity. It isnot a simple task to determine when to activate the feed back loop, butone safe clue is the activity from the user's own voice. As mentionedearlier, this can be done in many different ways and it is not crucialto the invention which way is chosen here.

In an embodiment of the invention the sound entering the cavity from thetissue and causing the problematic sound levels in the cavity iscaptured by a viberation pick-up device. The viberation signal isfiltered in a filter and combined with the signal which is captured bythe external microphone or input line of the device. In this way thecause of the occlusion problem, namely the sound conducted into the earcanal from the surrounding tissue, is used in a direct feed forwardmanner to eliminate or reduce the low frequency sound built up in thecavity. processor amplify the low frequency part of the signal from theexternal microphone in order to compensate for the attenuation of theuseful part of the signal from the external microphone or input line. Inthis way the useful low frequency parts of the signal, which areattenuated by the feed back loop, may be restored in the signalprocessor. Thus the user gets the sound from the surroundings with theusual amplification while the occlusion effect is removed or reduced.

According to an embodiment of the invention feed back loop from theadditional microphone is activated by a user's own voice activity. It isnot a simple task to determine when to activate the feed back loop, butone safe clue is the activity from the user's own voice. As mentionedearlier, this can be done in many different ways and it is not crucialto the invention which way is chosen here.

In an embodiment of the invention the sound entering the cavity from thetissue and causing the problematic sound levels in the cavity iscaptured by a vibration pick-up device. The vibration signal is filteredin a filter and combined with the signal which is captured by theexternal microphone or input line of the device. In this way the causeof the occlusion problem, namely the sound conducted into the ear canalfrom the surrounding tissues is used in a direct feed forward manner toeliminate or reduce the low frequency sound built up in the cavity.

In a further embodiment of the invention an inward pointing microphonemonitors the sound pressure in the cavity. This signal is compared withthe signal from the external microphone or input line, and where thecomparison result is used to control the shape of the filter. In thisway it is assured that the sound inside the ear canal is not allowed tobecome elevated due to sounds transmission through the tissue of theuser and into the ear canal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch of the feed back approach to anti-occlusion with aninternal microphone according to the invention.

FIG. 2 shows a sketch of the feed forward control approach toanti-occlusion using a vibration pick-up.

FIG. 3 is a schematic representation of the vibration pick-up.

DESCRIPTION OF A PREFERRED EMBODIMENT

The system of FIG. 1 comprises a microphone block comprising microphone1, AD-converter AD and transfer function HEM. The system furthercomprises block 2 comprising hearing aid block HHA, additional digitalblock HC and DA-converter DA. An internal microphone 8 is used in aconventional feed back control system as sketched in FIG. 1. Note thatthe control loop 9 is assumed to be formed in the analog domain. This isreflected in symbols for the receiver 3 H_(Ta) and internal microphone 8H_(Ma), transfer functions, where the subscript a denotes a transferfunction between two analog signals. Furthermore, the transfer functionof the analog feed back controller is denoted by D_(a) and finally anadditional digital block H_(C) has been added after the hearing aidblock H_(HA), as a means of correcting the changes to the amplificationcharacteristic of the hearing aid introduced by the feed back controlsystem. In this set-up H_(C) also includes the conversion from discretetime signal to analogue signal.

The relation between the source variables and the pressure at theeardrum P_(ED) is

$\begin{matrix}{p_{ED} = {\frac{\begin{matrix}{{p_{ES}H_{EM}H_{HA}H_{C}H_{Ta}} + q_{OV}} \\{( {Z_{FC} + {Z_{AC}H_{EM}H_{HA}H_{C}H_{Ta}}} ) + {q_{A}Z_{A}}}\end{matrix}}{1 + {D_{a}H_{Ta}H_{Ma}}}.}} & (1)\end{matrix}$

From this equation it is seen that the amount of attenuation, which thebody-conducted terms will be subject to is determined by the denominator1+D_(a)H_(Ta)H_(Ma). Thus, the design of the controller D_(a) willdepend on the desired performance, but is otherwise only dependent onthe combined physical transfer function H_(Ta)H_(Ma)—in the followingshortened to H_(TMa). It is also seen that desired sound p_(ES) will besubjected to the same attenuation as the body-conducted terms. However,this can be counteracted by setting H_(C)=1+D_(a)H_(TMa), which in turnwill mean that the second term in the parantheses following q_(OV) alsowill assume its original un-controlled value.

Since H_(TMa) will vary considerably between individual subjects andover time on each individual user, the design of D_(a) will have to beadaptive. Thus, the system sketched in FIG. 1 will have to be extendedwith an adaptive model of H_(TM), which obviously will have to bedigital. Thus the anti-occlusion system will comprise an adaptivediscrete time observer from which the transfer function of the analoguecontroller will be designed. The resulting adjustments to the controllerstructure will then be implemented as a digitally controllable analoguefilter.

In the feed back realization (FIG. 1), the filter D_(a) is designedaccording to the requested attenuation at low frequencies due toocclusion, but stability considerations must also be taken into account.Stability is ensured through analysis of the appropriate Nyquist curvefor the open loop case and subsequent gain and filtering adjustment.

The combination of signals from the feed back path and from the hearingaid block can be done by means of a receiver equipped with two seperatecoils in the electromagnetic system. Hence, as shown in EP patent 1 154673 the magnetic fields are added within the transducer.

If a vent is present in the hearing aid, the signal coming into thecavity through the vent will also be attenuated by the feed back system.If the vent has a large diameter it will in general decrease theocclusion effect and the anti-occlusion system will be adjustedaccordingly or in some cases removed entirely.

In another approach as seen in FIG. 2 an additional electroacoustictransducer 10 is used, which can pick up the vibrations of the softtissue in the ear canal 5, without picking up either the external soundpressure or the sound pressure generated in the volume 6 between theearmould 4 and the eardrum 7. An idealized block diagram of the controlsystem using such a transducer is seen in FIG. 2. The system of FIG. 2comprises a microphone block comprising microphone 1, AD-converter ADand transfer function HEM. The system further comprises block 2comprising hearing aid block HHA and DA-converter DA.

As in the previous section the control is realised in the analoguedomain, which means that in this case H_(HA) includes the conversionfrom discrete time to continuous time. Further, the relations betweenthe own voice volume velocity, q_(OV), and the volume velocity of otherinternal sources, q_(A), and the signal picked up by the additionaltransducer 10 has been denoted by Z′_(FC) and Z′_(A), respectively, the(analogue) transfer function of the alternative transducer has beendenoted by H′_(Ma), and the controller by D′_(a). The relation betweenthe source signals and the sound pressure at the eardrum is

$\begin{matrix}{p_{ED} = {{p_{ES}H_{EM}H_{HA}H_{Ta}} + {q_{OV}( {Z_{FC} - {Z_{FC}^{\prime}H_{Ma}^{\prime}D_{a}^{\prime}H_{Ta}} + {Z_{AC}H_{EM}H_{HA}H_{Ta}}} )} + {{q_{A}( {Z_{A} - {Z_{A}^{\prime}H_{Ma}^{\prime}D_{a}^{\prime}H_{Ta}}} )}.}}} & (2)\end{matrix}$

It is seen that occlusion can be reduced by adjustment of the controllerD′_(a) so that the q_(OV) term is made sufficiently small.

In this idealised description it is clear that a couple of potentialsignal paths have been left out of the picture. The most important oneis the path from the sound pressure at the eardrum to the signal pickedup by the vibration pick-up. If this path is significant the system willbe a hybrid feed forward/feed back system, which will be more difficultto design. A probably much less important contribution is that from theexternal sound pressure to the signal picked up by the vibrationpick-up.

The above described feed forward approach may be supplemented by amicrophone 8 measuring the sound pressure in the cavity 6 as in the feedback approach (se FIG. 1). In relation to the feed forward approach, thetransducer provides an error signal used for dynamic adjustment of thecontroller D′_(a) for minimum deviation between sound pressure in thecavity and the desired signal at the eardrum which will probably be,p _(ED) =p _(ES) H _(EM) H _(HA) H _(Ta) +q _(OV) Z _(AC) H _(EM) H_(HA) H _(Ta)

Hence, the additional internal microphone makes it possible to use anadaptive filter approach taking changes in transfer functions throughhuman tissue into account. These changes could stem from facialexpressions, jaw movements, temperature changes etc.

The feed forward embodiment of the anti-occlusion system has atransducer which provides a measure of the tissue vibrations and sincethis vibration contribution is known, an equivalent signal can beemitted in opposite phase from the receiver in order to cancel theinfluence of this signal in the cavity.

The feed forward embodiment above may also be implemented using digitalsignal processing, such that the signal from the vibration pick-up isconverted with its own AD converter and the DA converter in FIG. 2instead becomes part of H_(Ta).

In another embodiment the above mentioned adaptive adjustment is notincluded and the internal microphone is not included.

In FIG. 3 a transducer 10 for picking up body conducted sound isoutlined. The transducer 10 is constructed on the basis of acylindrically shaped Knowles FG microphone 11. A Knowles FG3453-C with acut-off frequency of 125 Hz is used. The transducer 10 consists of amicrophone 11 equipped with an airtight cap 12 or bell of fluoriderubber. The rubber bell is 1.5 mm high measured from the top of themicrophone and 0.15 mm thick at the top. The design provides goodvibration sensitivity when suitable physical contact exists between therubber and the surrounding tissue. This choice represents a very compactand yet simple transducer, and it ensures good sensitivity and a highdegree of attenuation of air-borne sound. The transducer is mounted sothat good contact to the skin is provided while leaving sufficient airin the cavity 13 in front of the microphone in order to avoid rectifyingthe signal.

1. Method for counteracting the occlusion effect of an electronic devicedelivering an audio signal to an ear, wherein the electronic devicecomprises a transmission path with an external microphone or input linewhich receives a signal p_(ES) from the environment, a signal processorand a receiver which receives a processed signal from the signalprocessor and delivers sound signals to the ear, whereby an ear piece isinserted into the ear canal and totally or partially blocks the canalwhereby the sound conditions in a cavity between the ear piece and thetympanic membrane are directly or indirectly determined, and wheneverconditions leading to occlusion problems are determined, transmissioncharacteristic of the transmission path to the receiver counteracts theocclusion effect, monitoring the sound conditions in the cavity by anadditional microphone which is acoustically coupled to the cavity, usingthe signal from the additional microphone in a feed back loop to thereceiver in order to attenuate the low frequency part of the sound inthe cavity, and forming the feed back loop to the receiver in theanalogue domain.
 2. Method as claimed in claim 1, including monitoringconditions leading to occlusion to determine activity of the user's ownvoice, and when a user's own voice activity is detected, reducingamplification through the signal processor in the frequency region below1 kHz.
 3. Method as claimed in claim 1, including amplifying a lowfrequency part of the signal from the external microphone in the signalprocessor in order to compensate for the attenuation of a useful part ofthe signal from the external microphone or input line.
 4. Method asclaimed in claim 1, including activating the feed back loop from theadditional microphone by a user's own voice activity.
 5. Method asclaimed in claim 1, whereby the sound entering the cavity from thetissue and causing the occlusion sound levels within the cavity iscaptured by a vibration pick-up, and where the vibration signal isfiltered in a filter D′_(a) and combined with the signal which iscaptured by the external microphone or input line of the device. 6.Method as claimed in claim 5, including monitoring the sound pressure inthe cavity with an inward pointing microphone producing a signal,comparing said signal with the signal from the external microphone orinput line, and using the comparison to control the shape of the filterD′_(a).
 7. Method as claimed in claim 1, whereby the detection of auser's own voice activity is carried out by a vibration pick-up incontact with a body portion of the user.
 8. Method as claimed in claim1, wherein the transmission path comprises a conversion from discretetime signals to analogue signals to allow the feed back loop to thereceiver to be formed in the analogue domain.
 9. Method as claimed inclaim 1, wherein stability considerations are taken into account throughanalysis of the appropriate Nyquist curve for the open loop case andsubsequent gain and filtering adjustment.